Spin polarization concerns the degree to which the spin or intrinsic angular momentum of elementary particles is aligned with a given direction. This spin affects the conduction of electrons in ferromagnetic metals, such as iron, giving rise to spin polarized currents. Spin polarized current affects spin waves, which are propagating disturbances in the ordering of magnetic materials. Spin polarization also concerns spintronics, a branch of electronics concerning the intrinsic spin of an electron, its associated magnetic moment, and the electron's fundamental electronic charge. Spintronics devices concern Tunnel Magnetoresistance (TMR), which uses quantum-mechanical tunneling of electrons through a thin insulator to separate ferromagnetic layers, and Spin Torque Transfer (STT), where a current of spin polarized electrons may be used to control the magnetization direction of ferromagnetic electrodes.